(1) Field of the Invention
This invention relates to casting of metal ingots by direct chill (DC) casting techniques. More particularly, the invention relates to methods of and apparatus for reducing so-called butt curl that occurs in the formation of ingots during DC casting.
(2) Description of the Related Art
DC casting has been used for many years for producing metal ingots, particularly ingots made of aluminum and aluminum-based alloys. Such ingots are then often subjected to hot and cold rolling to produce metal sheet supplied to industry for the fabrication of products or parts thereof. Briefly described, DC casting involves continuously introducing molten metal into a water-cooled vertical-axis mold having the shape of the desired ingot so that the periphery of the metal quickly cools and becomes sufficiently strong to allow an embryonic ingot to be withdrawn from the opposite (lower) end of the mold supported on a descending bottom block that initially closes the lower end of the mold. To provide rapid cooling of the embryonic ingot as it emerges from the lower end of the mold, streams of a liquid coolant (normally water) are contacted with the external surface of the ingot immediately below the mold and the coolant flows down the outer surface of the ingot. A variation of this technique employs a horizontal-axis casting mold, but the procedure is essentially the same.
A problem that has been difficult to overcome in DC casting techniques, particularly when casting rectangular ingots, is so-called butt curl. This is a tendency of the bottom end of the ingot (the part formed first) to adopt a curved profile under the effects of thermal stresses that are produced at the start of casting. Such bowing causes the side and end faces of the ingot to buckle and distort adjacent to the lower end, although the effect is most pronounced on the short end faces of a rectangular ingot. The resulting distortion causes problems during subsequent rolling of the ingot and, to avoid this, the lower end part of the ingot may be cut off and discarded before rolling commences. This is wasteful of material and adds an additional step to the overall process.
Attempts have been made in the past to reduce or eliminate butt curl but without a satisfactory degree of success,
U.S. Pat. No. 3,441,079 to Bryson issued Apr. 29, 1969 discloses a method and apparatus for continuously casting aluminum ingots wherein the emergent ingot is subjected to controlled cyclic cooling to decrease the extent of bottom-bow (butt curl) and notch formation.
U.S. Pat. No. 3,713,479 also to Bryson issued Jan. 30, 1973 discloses a method and apparatus for direct chill casting in which the ingot emerging from the mold passes successively through a first cooling zone and a second cooling zone located at a predetermined distance from the first cooling zone along the direction of ingot advance. The purpose of this disclosure is primarily to allow faster casting speeds without causing hot cracking of the ingot.
U.S. Pat. No. 5,582,230 to Wagstaff et al. issued Dec. 10, 1996 discloses a method and apparatus in which two sets of coolant streams are discharged onto an ingot emerging from a direct chill casting mold. The streams are orientated at different angles. One set of the streams is used during the initial stage of casting and both are used in the main casting stage. The use of a single set of streams in the initial stage helps to reduce butt curl.
U.S. patent application publication no. 2002/0174971 A1 of Nov. 28, 2002 discloses a method and apparatus similar to that of Wagstaff et al. but in which the two sets of streams are blended so that a single stream is produced that can be varied in its point of impact with the ingot at different stages of casting to minimize cooling related defects in the ingot.
Despite these prior methods and apparatus improved solutions are desired.